Apparatus for digitalizing an image by analysis by means of a light beam

ABSTRACT

The carrier of the image to be analyzed is fixed on a transparent cradle shaped as a portion of cylinder; a light beam is sent back perpendicularly to the cradle by a mirror pivoting about an axis which blends with the axis of the cradle and which is moved with respect to said cradle in order to light up, successively, and line by line, the image elements of the image to be analyzed; the light beam after traversing the image carrier is sent back on to an optico-electrical device by means of an elliptical mirror, so as to return, without introducing any aberrations, the light issued from the different image elements of one line.

The present invention relates to an apparatus for digitalizing an imageby analysis by means of a light beam, and more particularly to anapparatus of the type comprising means for fixing the carrier of animage to be analyzed, at least one source of light, a first opticaldevice for projecting a light beam issued from said source onto theimage to be analyzed, means of relative displacement between the beamand the image carrier for successively lighting, line by line, theelements of image of the image to be analyzed, and a convertercomprising an optico-electrical device to convert in digital form theinformation representing the image elements.

Image digitalizing apparatus as well as apparatus used for restitutingdigitalized images are employed in particular for transmitting images inconnection with systems of teledetection by satellites.

It is a particular object of the invention to provide an apparatuspermitting the analysis of images carried by a transparent medium, suchas a film.

This object is reached with an apparatus of the aforedescribed type, inwhich according to the invention:

the fixing device comprises a transparent analysis cradle shaped as aportion of cylinder and designed to receive the image carrier,

the first optical device comprises a mirror rotating about an axis whichcoincides with that of the analysis cradle, said mirror receiving thelight beam and reflecting it perpendicularly to the surface of thecradle, and

a second optical device comprising an elliptic mirror in the focalpoints of which are placed the pivoting mirror and the optico-electricaldevice, receives the light which has gone through the analysis cradleand the image carrier, and sends back on a fixed photosensitive zone ofsaid optico-electrical device the light issued from the different imageelements of a same line.

Due to the use of an elliptic mirror, no aberration is introduced forall the image elements of every line.

Analysis of successive lines is achieved by means of a stepwise-movingdevice permitting the displacement of the cradle according to an axialtranslation, and with respect to the rotating mirror and to the opticaldevices. The reflecting surface of the elliptic mirror can thus berestricted to a relatively narrow strip of an elliptic surface, the onlyrequirement being that the width of such strip should allow thereflection of the line scanning beam. The construction of an ellipticmirror is therefore relatively simple and rather inexpensive.

According to a special feature of the apparatus according to theinvention, said apparatus further comprises a restitution cradle whichis separate from the analysis cradle and is designed to receive acarrier of an image to be restituted from information modulating thelight beam issued from the light source, both analysis and restitutingcradles being situated on the same cylindrical surface and being securedto one another, so that the means used for line scanning and for columnscanning the carrier of an image to be restituted are the same as thoseused for scanning the carrier of an image to be analyzed.

A device for restituting a digitalized image and a device for analyzingan image to be digitalized are thus combined within the same apparatus.

Further features and advantages of the apparatus according to theinvention will be more readily apparent on reading the followingdescription made with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatical general view of one embodiment of theapparatus according to the invention, and

FIG. 2 is a cross-sectional view along plane II--II of FIG. 1.

The apparatus illustrated in the drawings comprises a carriage 10 whichis slidable along a rectilinear slide 11a of a fixed horizontal table11. The movement of the carriage is controlled by a stepwise motor (notshown) driving in rotation a micrometric screw 12 in engagement with thecarriage 10.

The carriage 10 supports a structure 13 of general cylindrical shapewhose upper part constitutes a cradle 14 supported by two annularside-plates 15, 16 rising at the longitudinal ends of the carriage 10perpendicularly to the direction in which said carriage advances. Thecradle 14 is designed to receive an image carrier such as a film 20,carrying an image to be analyzed by transmission for subsequentdigitalizing. To this effect, the cradle 14 is made of transparentmaterial, such as glass for example. Its shape is that of a portion,situated between two generating lines, of a cylinder of circularcross-section of axis 17 parallel to the direction of movement of thecarriage 10. The concave face of the cradle 14 is turned so as to facetowards the table 11, and means, such as clamps (not shown) keep thefilm 20 against the outer convex face of the cradle.

A flat mirror 21 is mounted for rotating about axis 17. The mirror 21 isdriven by a motor 22 supported by the fixed table 11 by means of a bentarm 23. A light source 25 generates a light beam 26 which, eventuallyafter reflection onto a fixed plane mirror 27, goes through the centralopening of side-plate 16 and is propagated towards the mirror 21, itsaxis being the axis 17. The reflecting surface of mirror 21 is inclinedto form an angle of 45° with axis 17 so that the beam 26 is reflectednormally to the cylindrical surface of cradle 14.

FIG. 1 shows the carriage 10 at one end of its run on table 11. Themirror 21 is then located in the vicinity of side-plate 15 in order tobe in a position to reflect the beam 26 close to one of the transverseedges of cradle 14. Analysis of the image carried by the film 20 isachieved by moving the carriage 10 stepwise from one end of its run tothe other. In a manner known per se, the rotation of mirror 21 and theforward movement of carriage 10 are synchronized so that, for everystable position of the carrier 10 after one step forward, the mirror 21makes a rotation during which it lights up a complete line of the imageto be analyzed, i.e. a narrow transverse strip of the film 20 fixed onthe cradle 14. Whilst carriage 10 moves from the position shown in FIG.1, the motor 22 and, optionally, at least part of the horizontal branchof the arm 23, penetrate through the central opening of side-plate 15.

The light beam 26 obtained after the light has traversed the transparentcradle 14 and the film 20 is reflected by a mirror 29 in the directionof an optico-electrical device 30. Said mirror 29 is an elliptic mirror,namely with a reflecting surface situated on a portion of surface of astraight cylinder having an ellipse as a base, the axis of said cylinderbeing parallel to the axis 17. FIG. 2 is a cross-section through atransverse median plane of mirror 29. The cross-section of the mirror isa portion of ellipse E whose focal points are the center of the mirror21 situated on axis 17 and the center of the photosensitive surface ofthe optico-electric device 30. FIG. 2 also shows that the mirror 29 hasa reflecting surface defined by the dihedron having axis 17 as an edgeand containing the longitudinal edges of the cradle 14. Thus, the mirror29 sends back the beam 28 right along a complete line scanning of film20. In the longitudinal direction, the mirror 29 has a constant width atleast equal to the diameter of beam 28. Thus, it is possible to limitthe reflecting surface of the mirror to a relatively narrow strip,therefore rather simple and inexpensive to produce despite its ellipticshape. The mirror 29 is placed above the cradle 14, plumb with theposition of the mirror 21 and is supported by means of brackets 24secured to the table 11.

The apparatus described hereinabove works as follows:

Analysis in black and white is performed with a laser source. Theintensity of the laser beam is modulated when traversing the filmcarrying the image to be analyzed and is converted into an analogelectric signal by means of the optico-electrical device 30 which can bea photodiode-type element or a photomultiplier tube. Said signal issampled and converted to digital form by means of an analog-to-digitalconverter 31. The sampling rate is determined in connection with thespeed of rotation of mirror 21 for the desired number of digitalizedimage elements (pixels) for every line of the analyzed image. The use ofan elliptic mirror 29 prevents the introduction of any aberration duringa complete line scanning, the light issued from the different elementsof image being sent back to a unique point. The digital information wordoutputted from the converter represents the grey level of the imageelements.

An adjustment of the size of the analyzed element of image is possibleby using a diaphragm 32 (FIG. 2) in front of the photosensitive surfaceof the optico-electrical device 30, the size of said diaphragm definingthat of image element P (see FIG. 2). It is possible to provide aplurality of pre-calibrated diaphragms in order to vary the sizes of theimage elements in a stepwise way (for example from 10 to 100 microns).Obviously, the diameter of the beam used, at the level of cradle 14,should be at least equal and preferably greater than the size selectedfor the image element; a wide slightly convergent beam can be used tothis effect. The length of the advancing step of the carriage is alsoadjusted according to the dimension of the image elements. The samehighly precise quartz-stabilized oscillator is used for controlling thesampling rate of the converter and the speeds of the motors driving thecarriage 10 and the mirror 21.

Color analysis is possible with three laser sources, a red, a green anda blue for example, and whose beams are superposed to simultaneouslylight the same image element. Separation of the different beams isachieved by placing behind the diaphragm of the optico-electrical devicea color separator which uses for example dichroic filters. The separatedbeams are directed on photo-receiving means (photodiodes orphotomultipliers) each affected to a particular color and associated toa particular analog-to-digital converter. Thus are simultaneouslyobtained the digital information words which represent the analysis ofan image element in each basic color.

As a variant, only one photo-receiving member may be used by switchingthe different sources of light; analysis is then performed by scanningeach line three times in succession, i.e. once for every separate colorbeam.

According to a special feature of the apparatus according to theinvention, said apparatus also comprises a restitution cradle designedto receive a carrier for an image to be restituted from digitalinformation. In the illustrated example, the restitution cradle 18 issituated at the lower part of the cylindrical structure 13, cradles 14and 18 being substantially symmetrical together with respect to axis 17.Accordingly, the shape of cradle 18 is that of a portion taken betweentwo generating lines of the same cylinder of circular cross-section asthat according to which is placed cradle 14, the concave face of cradle18 being turned towards mirror 21. The carrier for the image to berestituted, for example a blank film, is fixed on the concave face ofcradle 18 by means such as clamps (not shown). Contrary to the cradlefor analysis by transmission, the restitution cradle 18 is produced froman opaque material.

The position of cradle 18 makes it possible to use for the restitutionthe same line scanning means (pivoting mirror) and column scanning means(stepwise driven carriage) as for the analysis. It is also possible touse the same source of light (black and white) or the same sources oflight (color) as for the analysis, except that the intensity of thelaser beam issued from the one or more source of light is modulated inrelation to the recorded digital information representing the image tobe restituted. To this effect, a modulator 19 (shown in broken lines inFIG. 1) is placed at the output of the source and is controlled by videosignals worked out from said digital information, the control of themodulator being synchronized with that of the motors driving the mirror21 and carriage 10. For restituting a color image, the modulated beamsissued from the three sources are superposed in order to be appliedsimultaneously on the same image element.

We claim:
 1. An apparatus for digitalizing an image carried by atransparent medium, comprising:(a) a transparent analysis cradle shapedas a portion of a cylinder for receiving a transparent medium carryingan image to be digitalized; (b) light source means; (c) a first opticaldevice for projecting a light beam emitted from the light source meansonto an image carried by a medium fixed on the analysis cradle, saidfirst optical device comprising a mirror which is rotatable about anaxis aligned with that of the analysis cradle and which is oriented forreceiving said light beam and reflecting said light beam perpendicularlyto the surface of the analysis cradle; (d) means for rotating the mirrorabout said axis; (e) a second optical device comprisingoptico-electrical means hving a photosensitive surface to convert lightreceived by said photosensitive surface into an electrical signal, andan elliptic mirror with a reflecting surface situated on a portion of asurface of a cylinder having an ellipse as a base and having an axisparallel to the axis of the analysis cradle, the center of the rotatablemirror and the center of the photosensitive surface being respectivelysubstantially located at the focal points of the elliptic mirror, saidelliptic mirror receiving the light beams which have gone through theanalysis cradle and an image carrying medium fixed thereon as therotatable mirror is rotated to scan one image line, and said ellipticmirror reflecting the received light beam onto the photosensitivesurface so as to convert successive image elements of said image lineinto respective electrical signals; (f) means for digitalizing saidelectrical signals; and (g) means for moving the analysis cradle intranslation with respect to the rotatable mirror and second opticaldevice so that an image carrying medium fixed on the analysis cradle isscanned line by line.
 2. An apparatus as claimed in claim 1, whereinsaid analysis cradle is movable stepwise in translation with respect tothe rotatable mirror and second optical device.
 3. An apparatus asclaimed in claim 1, wherein the second optical device further comprisesa diaphragm placed in front of the photosensitive surface to determinethe size of the analyzed image elements.
 4. An apparatus as claimed inclaim 1, further comprising a restitution cradle shaped as a portion ofa cylinder for receiving a carrying medium for an image to berestituted, and means for modulating the light beam issued from saidlight source means, said restitution cradle and said analysis cradlehaving a common axis and being carried by a common support, whereby animage can be restituted by scanning an image carrying means fixed on therestitution cradle with the modulated beam by means of: (i) saidrotatable mirror; and (ii) said means for moving the analysis andrestitution cradles in translation with respect to the rotatable mirror.